1. Field of the Invention
The present invention relates to ion-exchange materials and, more particularly, to the in-situ polymerization of a cationic ion-exchange material in a solution of thermoplastic matrix resin.
2. Description of the Prior Art
The need to comply with ever-tightening environmental standards dictates that serious consideration be given to novel techniques for removing and/or recovering harmful contaminants from our waters. Chromium is one of these contaminants which has been classified as a toxic pollutant and, consequently, should not be discharged into waterways. Because of its widespread use in the plating industry and as a corrosion inhibitor in cooling tower systems, chromium is ubiquitous in the environment and its recovery would result in economic returns as well as producing a positive environmental impact.
Nitrate has been established as one of the ten inorganic chemicals designated by the National Interim Primary Drinking Water Regulations for systematic analysis in all public water supplies (effective June 1977). Nitrate is recognized as a noxious contaminant in water not only because it is a rich nutrient for biological growth such as algae blooms but because it is toxic to humans and animals resulting in methemoglobinemia, a disease effecting the oxygen carrying capacity of the blood hemoglobin. Infants are extremely susceptible (blue babies).
With the discovery that nitrosamines may be toxigenic and even carcinogenic, another dimension has been added to the chronic effects of nitrate. Nitrosamines are formed by the interaction between nitrites and secondary or tertiary amines under conditions similar to those found in the mammalian stomach. Nitrates are normally removed from waters by anaerobic denitrification, ion-exchange, electrodialysis, and reverse osmosis. Activated carbon is not effective as a scavenger for nitrate since it leaves high residual nitrate in the effluent.
Since ion-exchange hollow fibers require little pumping cost and no regeneration, it is conceivable that highly stable hollow fibers could provide a cost-effective means of simultaneously removing these contaminants and possibly even concentrating them for subsequent fertilizer usuage.
Current technology for removal of ions from dilute streams is largely oriented to the use of conventional packed, ion-exchange beds. These processes, however, have their problems. There is, for example, significant current effort toward the development of macroreticular pores in the ion-exchange beads which would be less susceptible to irreversible clogging. There are problems in the preparation of beads which have adequate porosity but which are still not unduly fragile. In the preparation of commercial ion-exchange beads, the process is as follows:
A cross-linked polymer bead is formed by reacting, for example, styrene and divinylbenzene. The percentage of cross-linker (divinylbenzene) determines the extent of swelling in the final bead as ions are exchanged. The greater the percentage of cross-linker, the less the swelling. Concurrently, the greater the level of cross-linker, the slower will be the diffusion of exchanging ions into and out of the beads, and the slower will be the process.
After the bead is formed, a chemical reaction such as sulfonation or chlormethylation is used to form the ion-exchange sites. From the description it is apparent that there are conflicting demands: high cross-link density helps stability but reduces product rate. Similarly, high ion-exchange capacity from the second step induces large swelling excursions, but provides greater capacity. Swelling of the resin beads occurs due to the osmotic pressures which are generated when the beads are exposed to different concentrations of various electrolytes. Pressure drop build-up is irregular and troublesome in regeneration processes. The choice of operating cycles is not straightforward at all and the beads are not inexpensive.
An alternative exists in semipermeable flat membranes but the technology is still in its infancy and the cost to efficiency ratio of membrane processes is not very satisfactory. Ion-exchange membranes offer significant advantages in separation processes with respect to ion-exchange resin beads. When the ion-exchange resins are in the form of membranes, they can be in contact with the solution to be separated and the stripping solution simultaneously and the ion-exchange process can be continuous rather than cyclic.
Ion-exchange membranes cannot be manufactured by the same techniques utilized to form ion-exchange beads since the swelling resulting from the formation of the ion-exchange site is too great to be borne by membranes which have a low degree of cross-linking. However, if the degree of cross-linking is raised, the membrane is too brittle to be useful. Most flat ion-exchange membranes are formed by first forming ion-exchange beads and then milling the beads into a thermoplastic resin as a binder for the resin structure. In a more recent process, the thermoplastic resin is milled in the presence of a swelling agent which is then replaced with a graftable ionic monomer. After grafting, the ionic site is bound to the membrane. The mechanical requirements are satisfied by using relatively thick sheets, in the range of 100-300 microns.
Anionic exchange hollow fibers have not been reported. Sulfonic acid cationic exchange type of hollow fibers have been prepared by irradiating polyethylene hollow fibers, immersing the irradiated fibers in styrene and heating the mixture to effect grafting. The fibers are then swollen in dichloromethane and sulfonated with chlorsulfonic acid, followed by hydrolysis. This procedure requires several steps, effects a random ion-exchange capacity and is limited to special reactants. Post-treatment of hollow fibers is further limited since the very small cross-section of the fibers and the fine porosity of the walls prevents introduction of preformed polymers into the bore or impregnation into the walls.
Ion-exchange fibers are disclosed in U.S. Pat. No. 3,944,485 in which monomers are extruded into the bore of a hollow fiber and into the pores of the wall and react therein to form insoluble ion-exchange material. This process which is practiced on preformed fibers, affects the permeability thereof and is unduly complicated and expensive.